Epoxides examples

A remote methoxy substituent directs maleic anhydride-mediated allylation predominantly in the 1,4-sense, comparable to the phenyltin-vinyl epoxide example (equation 244).139... 

In both epoxidation examples, the stereoselectivity is due to the cyclic nature of the transition state the fact that there is a hydrogen bond or O-metal bond delivering the reagent to one face of the alfcene. This is a very important concept, and we revisit it in the next chapter cyclic transition states are the key to getting good stereoselectivity in reactions of acyclic compounds. 

Ketone dianions react smoothly with a variety of carbonyl compounds and epoxides. Examples are listed in Table 3. 

Ionization methods leading to unstabilized cations can give pieparatively useful yields when the center is tertiary. In Scheme 17 a variety of examples is shown. Cyclohexene epoxides (example 2, Scheme 17) are generally useful initiators for monocyclizations. The cyclopropyl ketone is effective with aryl termination (example 4, Scheme 17), but in example 5 (Scheme 17) the ion obtained on cyclization is quenched by intramolecular attack of the enol. 

As indicated in the next section, there are several different types of epoxy resins. One type, classified as aliphatic epoxides, by definition do not feature an aromatic ring. The term cured epoxide as used in Fig. 2 refers to a resin formed from a bisphenol A-type epoxide cured with a carboxylic acid. Polycarbonate polymers also have different chemical compositions. Similar to the epoxide example, for example, there are also aliphatic-based polycarbonate resins, with one of the most famous being a polymer used for optical lenses. The polycarbonate resin example provided in Fig. 2 is an aromatic polycarbonate, which includes the one based on the bis-phenol A moiety, commonly known as Lexan. 

Hydroxyl groups also participate as nucleophiles in a variety of other acid-catalyzed cyclizations of synthetic utility. One example is the Ritter reaction, which normally produces amides upon reaction of olefins or alcohols with nitriles. Cyclization occurs with 1,3-diols, hydroxy alkenes, or epoxides. Examples of each are given in Eqs. (74)-(76). 

Figure 10.5 Small changes made in the pyrrolizidine alkaloid acetylseneciphylline N-oxide by Oreina species. The epoxide example is not found in plants...

Figure 19.5 Epoxidation of cis- and terminal alkenes with ketone 13 epoxidation examples...

When the epoxide substrate is nonsymmetrical, then, in principle, two regioisomers can be formed (Figure 9.75). In both regioisomers, the stereochemistry of the incoming nucleophile and the OH is trans clearly both processes are fundamentally 5 2 in character and go with inversion of stereochemistry. With good nucleophiles, under basic conditions, the reaction is straightforward as in most 5 2 reactions, attack takes place predominantly or exclusively at the less hindered of the two carbon atoms of the epoxide. Examples are shown in Figure 9.76. 

The stereoselective epoxidation examples in Table 5.1 and Fig. 5.1 are rare examples where the metal-mediated process (Path 2 or 3) clearly competes with... 

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